386 B. N. TEACH AND J. HORNE ON THE 



veins — are closely allied to each other in chemical composition. The granites 

 of Rooeness and Sandsting, the spherulitic felsite of Papa Stour, and the 

 rhyolites of Papa Little, yield similar results. The percentage of silica in the 

 Sandsting granite is 7096, in the Papa Stour felsite 69*12, in the grey and 

 pink varieties of rhyolite 7370 and 72 32 respectively. There is no appreci- 

 able difference in the other ingredients, save in the quantity of potash in the 

 Papa Stour felsite, which amounts to 1017 per cent. The specific gravity of 

 the granites and rhyolites is 2*6, while that of the Papa Stour felsite is 2*5. 



In conclusion, we might thus briefly summarise the results of these inves- 

 tigations. 



1. The porphyrite and diabase lavas of Shetland belong to a basic series, and 

 present microscopic characters akin to the great volcanic series of Lower Old 

 Red Sandstone age in central Scotland. 



2. The intrusive diabase dykes and bosses resemble the foregoing in chemi- 

 cal composition and microscopic characters, though they are separated from each 

 other by a considerable interval of time. 



3. The great intrusive sheets of Rooeness Hill and Sandsting consist of 

 binary granite and ordinary micaceous granite respectively, while the Papa 

 Stour sheet is composed of pink spherulitic felsite. The dykes of devitrified 

 rhyolite, associated with the binary granites, closely resemble the granites and 

 felsites in chemical composition, and hence the divergence in lithological and 

 microscopic characters is due to the different conditions under which they con- 

 solidated. 



4. The well-marked fluxion structure displayed by the rhyolites seems to 

 indicate that they originally possessed a glassy ground mass, which has to a 

 large extent disappeared through devitrification. 



5. The mere presence of fluxion structure in igneous rocks does not by 

 itself prove that the lavas were ejected at the surface, as the physical relations 

 of the Shetland rhyolites clearly show that they are intrusive. 



Note. — Our best thanks are due to Professor Renard and Mr T. Davies for the assistance they 

 kindly rendered in the microscopic examination of these rocks. 



